1. Field of the Invention
This invention relates to an improved process for making rare earth-barium-copper oxide superconductors with transition temperatures above 90 K.
2. Description of Related Art
Bednorz and Muller, Z. Phys. B64, 189-193 (1986), disclose a superconducting phase in the La-Ba-Cu-O system with a superconducting transition temperature of about 35 K. Samples were prepared by a coprecipitation method from aqueous solutions of Ba-, La- and Cu-nitrate in their appropriate ratios. An aqueous solution of oxalic acid was used as the precipitant.
Chu et al., Phys. Rev. Lett. 58, 405-407 (1987), report detection of an apparent superconducting transition with an onset temperature above 40 K under pressure in the La-Ba-Cu-O compound system synthesized directly from a solid-state reaction of La.sub.2 O.sub.3, CuO and BaCO.sub.3 followed by a decomposition of the mixture in a reduced atmosphere. Chu et al., Science 235, 567-569 (1987), disclose that a superconducting transition with an onset temperature of 52.5 K has been observed under hydrostatic pressure in compounds with nominal compositions given by (La.sub.0.9 Ba.sub.0.1).sub.2 CuO.sub.4-y, where y is undetermined. They state that the K.sub.2 NiF.sub.4 layer structure has been proposed to be responsible for the high-temperature superconductivity in the La-Ba-Cu-O system (LBCO). They further state that, however, the small diamagnetic signal, in contrast to the presence of up to 100% K.sub.2 NiF.sub.4 phase in their samples, raises a question about the exact location of superconductivity in LBCO.
Cava et al., Phys. Rev. Lett. 58, 408-410 (1987), disclose bulk superconductivity at 36 K in La.sub.1.8 Sr.sub.0.2 CuO.sub.4 prepared from appropriate mixtures of high purity La(OH).sub.3, SrCO.sub.3 and CuO powders, heated for several days in air at 1000.degree. C. in quartz crucibles. Rao et al., Current Science 56, 47-49 (1987), discuss superconducting properties of compositions which include La.sub.1.8 Sr.sub.0.2 CuO.sub.4, La.sub.1.85 Ba.sub.0.15 CuO.sub.4, La.sub.1.8 Sr.sub.0.1 CuO.sub.4, (La.sub.1-x Pr.sub.x).sub.2-y Sr.sub.y CuO.sub.4, and (La.sub.1.75 Eu.sub.0.25)Sr.sub.0.2 CuO.sub.4. Bednorz et al., Europhys. Lett. 3, 379-384 (1987), report that susceptibility measurements support high-T.sub.c superconductivity in the Ba-La-Cu-O system. In general, in the La-Ba-Cu-O system, the superconducting phase has been identified as the composition La.sub.1-x (Ba,Sr,Ca).sub.x O.sub.4-y with the tetragonal K.sub.2 NiF.sub.4 -type structure and with x typically about 0.15 and y indicating oxygen vacancies.
Wu et al., Phys. Rev. Lett. 58, 908-910 (1987), disclose a superconducting phase in the Y-Ba-Cu-O system with a superconducting transition temperature between 80 and 93 K . The compounds investigated were prepared with nominal composition (Y.sub.1-x Ba.sub.x).sub.2 CuO.sub.4-y and x=0.4 by a solid-state reaction of appropriate amounts of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO in a manner similar to that described in Chu et al., Phys. Rev. Lett. 58, 405-407 (1987). Said reaction method comprises more specifically heating the oxides in a reduced oxygen atmosphere of 2.times.10.sup.-5 bars (2 Pa) at 900.degree. C. for 6 hours. The reacted mixture was pulverized and the heating step was repeated. The thoroughly reacted mixture was then pressed into 3/16 inch (0.5 cm) diameter cylinders for final sintering at 925.degree. C. for 24 hours in the same reduced oxygen atmosphere. The material prepared showed the existence of multiple phases.
Hor et al., Phys. Rev. Lett. 58, 911-912 (1987), disclose that pressure has only a slight effect on the superconducting transition temperature of the Y-Ba-CuO superconductors described by Wu et al., supra.
Sun et al., Phys. Rev. Lett. 58, 1574-1576 (1987), disclose the results of a study of Y-Ba-Cu-O samples exhibiting superconductivity with transition temperatures in the 90 K range. The samples were prepared from mixtures of high-purity Y.sub.2 O.sub.3, BaCO.sub.3 and CuO powders. The powders were premixed in methanol or water and subsequently heated to 100.degree. C. to evaporate the solvent. Two thermal heat treaments were used. In the first, the samples were heated in Pt crucibles for 6 hours in air at 850.degree. C. and then for another 6 hours at 1000.degree. C. After the first firing, the samples were a dark-green powder, and after the second firing, they became a very porous, black solid. In the second method, the powders were heated for 8-10 hours at 1000.degree. C., ground and then cold pressed to form disks of about 1 cm diameter and 0.2 cm thickness. The superconducting properties of samples prepared in these two ways were similar. X-ray diffraction examination of the samples revealed the existence of multiple phases.
Cava et al., Phys. Rev. Lett. 58, 1676-1679 (1987), have identified this superconducting Y-Ba-Cu-O phase to be orthorhombic, distorted, oxygen-deficient perovskite YBa.sub.2 Cu.sub.3 O.sub.9-.delta. where .delta. is about 2.1, and have presented the X-ray diffraction powder pattern and lattice parameters for the phase. The single-phase YBa.sub.2 Cu.sub.3 O.sub.9-.delta. was prepared in the following manner. BaCO.sub.3, Y.sub.2 O.sub.3 and CuO were mixed, ground and then heated at 950.degree. C. in air for 1 day. The material was then pressed into pellets, sintered in flowing O.sub.2 for 16 hours and cooled to 200.degree. C. in O.sub.2 before removal from the furnace. Additional overnight treatment in O.sub.2 at 700.degree. C. was found to improve the observed properties.
Takita et al., Jpn. J. Appl. Phys. 26, L506-L507 (1987), disclose the preparation of several Y-Ba-Cu compositions with superconducting transitions around 90 K by a solid-state reaction method in which a mixture of Y.sub.2 O.sub.3, CuO, and BaCO.sub.3 was heated in an oxygen atmosphere at 950.degree. C. for more than 3 hours. The reacted mixture was pressed into 10 mm diameter disks for final sintering at 950.degree. or 1000.degree. C. for about 3 hours in the same oxygen atmosphere.
Takabatake et al., Jpn. J. Appl. Phys. 26, L502-L503 (1987), disclose the preparation of samples of Ba.sub.1-x Y.sub.x CuO.sub.3-z (x=0.1, 0.2, 0.25, 0.3, 0.4, 0.5, 0.6, 0.8 and 0.9) from the appropriate mixtures of BaCO.sub.3, Y.sub.2 O.sub.3 and CuO. The mixture was pressed into a disc and sintered at 900.degree. C. for 15 hours in air. The sample with x=0.4 exhibited the sharpest superconducting transition with an onset near 96 K.
Syono et al., Jpn. J. Appl. Phys. 26, L498-L501 (1987), disclose the preparation of samples of superconducting Y.sub.0.4 Ba.sub.0.6 CuO.sub.2.22 with T.sub.c higher than 88 K by firing mixtures of 4N Y.sub.2 O.sub.3, 3N BaCO.sub.3 and 3N CuO in the desired proportions. The mixtures were prefired at 1000.degree. C. for 5 hours. They were ground, pelletized and sintered at 900.degree. C. for 15 hours in air and cooled to room temperature in the furnace. They also disclose that almost equivalent results were also obtained by starting from concentrated nitrate solution of 4N Y.sub.2 O.sub.3, GR grade Ba(NO.sub.3).sub.2 and Cu(NO.sub.3).sub.2.
Takayama-Muromachi et al., Jpn. J. Appl. Phys. 26, L476-L478 (1987), disclose the preparation of a series of samples to try to identify the superconducting phase in the Y-Ba-CuO system. Appropriate amounts of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO were mixed in an agate mortar and then fired at 1173.+-.2 K for 48-72 hours with intermediate grindings. X-ray diffraction powder patterns were obtained. The suggested composition of the superconducting compound is Y.sub.1-x Ba.sub.x CuO.sub.y where 0.6&lt;x&lt;0.7.
Hosoya et al., Jpn. J. Appl. Phys. 26, L456-L457 (1987), disclose the preparation of various superconductor compositions in the L-Ba-Cu-O systems where L=Tm, Er, Ho, Dy, Eu and Lu. Mixtures of the proper amounts of the lanthanide oxide (99.9% pure), CuO and BaCO.sub.3 were heated in air. The obtained powder specimens were reground, pressed into pellets and heated again.
Hirabayashi et al., Jpn. J. Appl. Phys. 26, L454-L455 (1987), disclose the preparation of superconductor samples of nominal composition Y.sub.1/3 Ba.sub.2/3 CuO.sub.3-x by coprecipitation from aqueous nitrate solution. Oxalic acid was used as the precipitant and insoluble Ba, Y and Cu compounds were formed at a constant pH of 6.8. The decomposition of the precipitate and the solid-state reaction were performed by firing in air at 900.degree. C. for 2 hours. The fired products were pulverized, cold-pressed into pellets and then sintered in air at 900.degree. C. for 5 hours. The authors found that the the sample was of nearly single phase having the formula Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7. The diffraction pattern was obtained and indexed as having tetragonal symmetry.
Ekino et al., Jpn. J. Appl. Phys. 26, L452-L453 (1987), disclose the preparation of a superconductor sample with nominal composition Y.sub.1.1 Ba.sub.0.9 CuO.sub.4-y. A prescribed amount of powders of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO was mixed for about an hour, pressed under 6.4 ton/cm.sup.2 (14 MPa) into pellet shape and sintered at 1000.degree. C. in air for 3 hours.
Akimitsu et al., Jpn. J. Appl. PhYs. 26, L449-L451 (1987), disclose the preparation of samples with nominal compositions represented by (Y.sub.1-x Ba.sub.x).sub.2 CuO.sub.4-y. The specimens were prepared by mixing the appropriate amounts of powders of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO. The resulting mixture was pressed and heated in air at 1000.degree. C. for 3 hours. Some samples were annealed at appropriate temperatures in O.sub.2 or CO.sub.2 for several hours. The authors noted that there seemed to be a tendency that samples annealed in O.sub.2 showed a superconducting transition with a higher onset temperature but a broader transition than non-annealed samples.
Semba et al., Jpn. J. Appl. Phys. 26, L429-L431 (1987), disclose the preparation of samples of Y.sub.x Ba.sub.1-x CuO.sub.4-d where x=0.4 and x=0.5 by the solid state reaction of BaCO.sub.3, Y.sub.2 O.sub.3 and CuO. The mixtures are heated to 950.degree. C. for several hours, pulverized, and then pressed into disk shape. This is followed by the final heat treatment at 1100.degree. C. in one atmosphere O.sub.2 gas for 5 hours. The authors identified the phase that exhibited superconductivity above 90 K as one that was black with the atomic ratio of Y:Ba:Cu of 1:2:3. The diffraction pattern was obtained and indexed as having tetragonal symmetry.
Hatano et al., Jpn. J. Appl. Phys. 26, L374-L376 (1987), disclose the preparation of the superconductor compound Ba.sub.0.7 Y.sub.0.3 Cu.sub.1 O.sub.x from the appropriate mixture of BaCO.sub.3 (purity 99.9%), Y.sub.2 O.sub.3 (99.99%) and CuO (99.9%). The mixture was calcined in an alumina boat heated at 1000.degree. C. for 10 hours in a flowing oxygen atmosphere. The color of the resulting well-sintered block was black.
Hikami et al., Jpn. J. Appl. Phys. 26, L347-L348 (1987), disclose the preparation of a Ho-Ba-Cu oxide, exhibiting the onset of superconductivity at 93 K and the resistance vanishing below 76 K, by heating a mixture of powders Ho.sub.2 O.sub.3, BaCO.sub.3 and CuO with the composition Ho:Ba:Cu=0.246:0.336:1 at 850.degree. C. in air for two hours. The sample was then pressed into a rectangular shape and sintered at 800.degree. C. for one hour. The sample looked black, but a small part was green.
Matsushita et al., Jpn. J. Appl. Phys. 26, L332-L333 (1987), disclose the preparation of Ba.sub.0.5 Y.sub.0.5 Cu.sub.1 O.sub.x by mixing appropriate amounts of BaCO.sub.3 (purity 99.9%), Y.sub.2 O.sub.3 (99.99%) and CuO (99.9%). The mixture was calcined at 1000.degree. C. for 11 hours in a flowing oxygen atmosphere. The resultant mixture was then pulverized and cold-pressed into disks. The disks were sintered at 900.degree. C. for 4 hours in the same oxygen atmosphere. The calcined powder and disks were black. A superconducting onset temperature of 100 K was observed.
Maeno et al., Jpn. J. Appl. Phys. 26, L329-L331 (1987), disclose the preparation of various Y-Ba-Cu oxides by mixing powders of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO, all 99.99% pure, with a pestle and mortar. The powders were pressed at 100 kgf/cm.sup.2 (98.times.10.sup.4 Pa) for 10-15 minutes to form pellets with a diameter of 12 mm. The pellets were black. The heat treatment was performed in two steps in air. First, the pellets were heated in a horizontal, tubular furnace at 800.degree. C. for 12 hours before the heater was turned off to cool the pellets in the furnace. The pellets were taken out of the furnace at about 200.degree. C. About half the samples around the center of the furnace turned green in color, while others away from the center remained black. The strong correlation with location suggested to the authors that this reaction occurs critically at about 800.degree. C. The pellets were then heated at 1200.degree. C. for 3 hours and then allowed to cool. Pellets which turned light green during the first heat treatment became very hard solids whereas pellets which remained black in the first heat treatment slightly melted or melted down. Three of the samples exhibited an onset of superconductivity above 90 K.
Iguchi et al., Jpn. J. Appl. Phys. 26, L327-L328 (1987), disclose the preparation of superconducting Y.sub.0.8 Ba.sub.1.2 CuO.sub.y by sintering a stoichiometrical mixture of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO at 900.degree. C. and at 1000.degree. C. in air.
Hosoya et al., Jpn. J. Appl. Phys. 26, L325-L326 (1987), disclose the preparation of various superconducting specimens of the L-M-Cu-O systems where L=Yb, Lu, Y, La, Ho and Dy and M=Ba and a mixture of Ba and Sr by heating the mixtures of appropriate amounts of the oxides of the rare earth elements (99.9% pure), CuO, SrCO.sub.3 and/or BaCO.sub.3 in air at about 900.degree. C. Green powder was obtained. The powder samples were pressed to form pellets which were heated in air until the color became black.
Takagi et al., Jpn. J. Appl. Phys. 26, L320-L321 (1987), disclose the preparation of various Y-Ba-Cu oxides by reacting mixtures containing the prescribed amounts of powders of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO at 1000.degree. C., remixing and heat-treating at 1100.degree. C. for a few to several hours. An onset temperature of superconductivity at 95 K or higher was observed for a specimen with the nominal composition of (Y.sub.0.9 Ba.sub.0.1)CuO.sub.y.
Hikami et al., Jpn. J. Appl. Phys. 26, L314-L315 (1987), disclose the preparation of compositions in the Y-Ba-CuO system by heating the powders of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO to 800.degree. C. or 900.degree. C. in air for 2-4 hours, pressing into pellets at 4 kbars (4.times.10.sup.5 Pa) and reheating to 800.degree. C. in air for 2 hours for sintering. The samples show an onset of superconductivity at 85 K and a vanishing resistance at 45 K.
Bourne et al., Phys. Letters A 120, 494-496 (1987), disclose the preparation of Y-Ba-Cu-O samples of Y.sub.2-x Ba.sub.x CuO.sub.4 by pressing finely ground powders of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO into pellets and sintering the pellets in an oxygen atmosphere at 1082.degree. C. Superconductivity for samples having x equal to about 0.8 was reported.
Moodenbaugh et al., Phys. Rev. Lett. 58, 1885-1887 (1987), disclose superconductivity near 90.degree. K. in multiphase samples with nominal composition Lu.sub.1.8 Ba.sub.0.2 CuO.sub.4 prepared from dried Lu.sub.2 O.sub.3, high-purity BaCP.sub.3 (presumably BaCO.sub.3), and fully oxidized CuO. These powders were ground together in an agate mortar and then fired overnight in air at 1000.degree. C. in Pt crucibles. This material was ground again, pelletized, and then fired at 1100.degree. C. in air for 4-12 hours in Pt crucibles. Additional samples fired solely at 1000.degree. C. and those fired at 1200.degree. C. show no signs of superconductivity.
Hor et al., Phys. Rev. Lett. 58, 1891-1894 (1987), disclose superconductivity in the 90 K range in ABa.sub.2 Cu.sub.3 O.sub.6+x with A=La, Nd, Sm, Eu, Gd, Ho, Er, and Lu in addition to Y. The samples were synthesized by the solid-state reaction of appropriate amounts of sesquioxides of La, Nd, Sm, Eu, Gd, Ho, Er, and Lu, BaCO.sub.3 and CuO in a manner similar to that described in Chu et al., Phys. Rev. Lett. 58, 405 (1987) and Chu et al., Science 235, 567 (1987).